Detrital zircon ages and Hf isotopic compositions of metasedimentary rocks in the Wuqia area of Southwest Tianshan, NW China: implications for the early Paleozoic tectonic evolution of the Tianshan orogenic belt

The Chinese Southwest Tianshan Orogenic Belt is located along the boundary between the Central Asian Orogenic Belt (CAOB) and the Tarim Block (TB), NW China. It records the convergence of the Tarim Block and the Middle Tianshan, and is, therefore, a crucial region for understanding the Eurasia continental growth and evolution. The Wulagen (geographical name) metasedimentary rocks of the Wuqia area (mainly metamorphic sandstones and mica schists) form one of the metamorphic terranes in the Southwestern Tianshan Orogenic Belt. The geochronology of these rocks is poorly known, which hampers our understanding of the tectonic evolution of the belt. We analyzed 517 zircon grains for detrital zircon U–Pb dating and 93 zircon grains for in situ Lu–Hf isotopic compositions from the Wulagen metasedimentary rocks. The analyzed zircon grains yield Neoarchean to late Paleozoic U–Pb ages with major age peaks at ~2543 Ma, 1814 Ma, 830 Ma, 460 Ma, and the youngest cluster of zircon (magmatogene) ages is 395 Ma. The zircon U–Pb data show that the late Paleozoic (Early Devonian) is the maximum depositional age of the Wulagen metasedimentary rocks, rather than the previously considered Precambrian period. The zircons with Paleozoic ages yield εHf(t) values of −22.0 to +11.3 and two-stage model ages (TDM₂) of 3.95 to 1.30 Ga, suggesting that the parental magmas were formed from partial melting of pre-existing crustal rocks. Our zircon U–Pb geochronology and Hf isotopic data indicate the major source regions for the Wulagen metasedimentary rocks was the Kyrgyzstan North Tianshan. The zircon age population of 600–400 Ma (peak at ~460 Ma) has negative εHf(t) values (−15.0 to −0.6) and Mesoproterozoic two-stage model ages, suggesting that the early Paleozoic magmatism resulted mainly from the melting of ancient crust, which played an important role in crustal evolution in the southern CAOB.

中国西南天山造山带位于中国西北的中亚造山带（Central Asian Orogenic Belt, CAOB）与塔里木地块（Tarim Block, TB）的交界地带，记录了塔里木地块与中天山的汇聚过程，因此是探究欧亚大陆地壳生长与演化的关键区域。乌恰地区乌拉根（地理名称）变沉积岩系（主要由变质砂岩与云母片岩组成）是西南天山造山带的变质地体之一。此前学界对该套岩石的年代学研究较为匮乏，这极大限制了我们对该造山带构造演化的认知。本次研究对乌拉根变沉积岩系中的517颗锆石开展碎屑锆石U-Pb定年（detrital zircon U–Pb dating），并对其中93颗锆石进行原位Lu-Hf同位素组成（in situ Lu–Hf isotopic compositions）分析。测试结果显示，锆石U-Pb年龄跨度为新太古代至晚古生代，主要年龄峰值集中于~2543 Ma、1814 Ma、830 Ma、460 Ma，最年轻的岩浆成因锆石年龄集群为395 Ma。锆石U-Pb数据表明，乌拉根变沉积岩系的最大沉积年龄为晚古生代（早泥盆世），而非此前认定的前寒武纪时期。古生代年龄的锆石其εHf(t)值介于-22.0至+11.3之间，两阶段模式年龄（two-stage model ages, TDM₂）为3.95 Ga至1.30 Ga，指示其母岩浆由先期存在的地壳岩石部分熔融形成。本次碎屑锆石U-Pb年代学与Hf同位素数据显示，乌拉根变沉积岩系的主要物源区为吉尔吉斯北天山。600~400 Ma年龄区间的锆石群体（峰值约460 Ma）具有负εHf(t)值（-15.0至-0.6）与中元古代两阶段模式年龄，表明早古生代岩浆作用主要源于古老地壳的熔融，这一过程对中亚造山带南部的地壳演化起到了重要作用。